This invention relates to improvements in power consumption of intermittently-operated traveling wave tube based amplifiers, particularly as used in satellite systems.
Amplifiers based on traveling wave tubes have numerous applications in wireless telecommunications and radar systems. Satellite communication is one class of applications, which, because of high efficiency and high reliability requirements, presents unique environmental constraints. On many satellites, traveling wave tube amplifiers, or TWTAs, are used to amplify low-level communication signals such that they can be transmitted to distant ground and/or space stations. For many satellite communication applications, TWTAs are the amplifier technology of choice because of their relatively high power and high conversion efficiency.
A satellite's cost is directly related to the amount of power and the thermal rejection capacity it must have. The amount of power required dictates the size and cost of the solar arrays, batteries and power conversion electronics that make up a satellite's power subsystem. The amount of thermal rejection capacity required dictates the size of the satellite's thermal radiators and the number of heat pipes, heaters and thermal blankets required in its thermal subsystem. In addition to the basic cost of these components, their mass directly affects cost of a satellite system since the cost of the required launch vehicle is directly related to the mass of the payload, such as a satellite.
FIG. 1 illustrates characteristics of the trajectory of an idealized non-geosynchronous orbiting (NGSO) satellite, which is a primary application of the present invention, showing a satellite 12 in positions A and B in line of sight with a tracking ground station 14 on earth 16. It will be noted that the satellite is in view of its target/source ground station(s) 14 for only a fraction F of the orbit T. Thus, power consumption reduction during non active times is significant in that it means that if communications equipment power consumption can be terminated during these periods, the satellite's required power and thermal system capacities can be reduced.
Although many ways of terminating communications equipment power consumption during these no-service periods of the orbit have been considered previously, none has been able to achieve desired substantial efficiencies. For example, in the past, several methods that have been used to reduce power consumption, primarily by disabling the TWTAs. The most obvious and straightforward is to turn it off. This achieves the intended result, but the electronic power conditioner (EPC) units used to power the typical TWTA (see FIG. 3 for a simplified TWTA block diagram comprising an EPC and TWT) are complex and sensitive devices, so that subjecting them to the tens of thousands of on/off cycles required during the life of a satellite mission is a significant reliability risk considered unacceptable by most satellite customers.
Disabling the pre-amplifier used to drive the TWTA is another technique used to disable these devices. Unfortunately this does not prevent the TWTA from amplifying spurious noise power. Amplification of noise is undesirable because it creates a source of interference that degrades the signal of other active satellites. Additionally, disabling the driver amplifier only marginally reduces the TWTA power consumption and thermal dissipation. Therefore a satellite using it would require larger than desired power and thermal system capacity and is therefore for reasons mentioned earlier a more expensive system.
Biasing anode voltage is yet another method of disabling TWTA operation and reducing satellite power and thermal resource requirements. A substantial bias potential for the anode (˜5700V for example) is required in order to be effective. Although possible to do, creating this high voltage signal is complex and expensive.
Since TWTAs contribute disproportionately to a satellite's power and thermal requirements—TWTAs typically consume around 90% of the power generated and distributed on a satellite and typically dissipate directly or indirectly about 70% of the heat that must be eliminated by the satellite's thermal control system—what is needed is a mechanism for providing more efficient implementations and designs for TWTA based amplifiers used in satellites, particularly NGSO satellite designs.